ESTIMATING EVAPOTRANSPIRATION IN PROCESSING TOMATOES

California growers presently use reference evapotranspiration (ETo) data and crop coefficient (Kc) values to estimate crop evapotranspiration (ETc) in processing tomatoes. However, a direct ETc measurement would be useful to investigate water management strategies that optimize fruit yield and quality for processing tomatoes. We evaluated the "surface renewal" method for estimating ETc in two processing tomato fields in northern California. The surface renewal sensible heat flux density data were calibrated against sonic anemometer measurements in each field. Surface renewal ETc was found to be similar to ETc estimated from ETo and Kc values during periods in the early and midseason when the crop was sufficiently irrigated. After the last irrigation, the crop underwent water stress, and directly measured ETc was less than estimated ETc. The difference between measured and estimated ET was proposed as an index to quantify water stress during deficit irrigation.